Automatic target acquisition in mti radar system

ABSTRACT

An airborne MTI radar system is disclosed for searching and tracking airborne targets over large bodies of water. In the search mode, returns from staggered PRF&#39;&#39;s are used to resolve range ambiguity of targets, and first and second multipath returns are used to more positively identify an airborne target for automatic acquisition. The number of target returns counted (1, 2 or 3) at each PRF combined with the numbers at the other two PRF&#39;&#39;s yields a plot combination count which serves to indicate the &#39;&#39;&#39;&#39;quality&#39;&#39;&#39;&#39; of a target in selecting new targets for tracking, but first each target is correlated with targets already being tracked. Uncorrelated targets are then checked as to quality by reordering the combination of target return counts in descending order and checking the resulting combination number, Qp, against a predetermined minimum acceptable plot quality, QM, for the range of the target and the sea state. If this check is passed by a particular target, it is entered into a table for automatic acquisition, provided the track store is not full to capacity; otherwise, the target is set up for display only for possible manual acquisition of track. Once acquired, the target is tracked automatically.

Waited gtate atet [191 King et al.

11] 3,31,174 Aug. 20, 1974 AUTOMATIC TARGET ACQUISITION IN MTI RADARSYSTEM [75] Inventors: Donald L. King, La Mirada; Gerald M. Goldberg,Chatsworth; nald P. Parke, Anaheim; Willis M. Priester, Garden Grove;Richard A. 1 Gebhardt, Orange, all of Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

[22] Filed: Feb. 5, 1973 [21] Appl. No.: 329,762

[52] 11.8. CI 343/7 A, 343/5 DP, 343/7.7

[51] Int. Cl. 601s 9/06 [58] Field of Search 343/5 DP, 7 A, 7.7

[56] References Cited UNITED STATES PATENTS 3,503,068 3/1970 Yamauchi343/5 DP X 3,646,554 2/1972 Fierston et al l 343/5 DP 3,731,304 5/1973Caspers et al. 343/5 DP Primary Examiner-T. H. Tubbesing Attorney,Agent, or Firm-W. H. MacAllister; Walter J. Adam [57] ABSTRACT Anairborne MTl radar system is disclosed for searching and trackingairborne targets over large bodies of water. In the search mode, returnsfrom staggered PRFs are used to resolve range ambiguity of targets, andfirst and second multipath returns are used to more positively identifyan airborne target for automatic acquisition. The number of targetreturns counted (1, 2 or 3) at each PRF combined with the numbers at theother two PRFs yields a plot combination count which serves to indicatethe quality of a target in selecting new targets for tracking, but firsteach target is correlated with targets already being tracked.Uncorrelated targets are then checked as to quality by reordering thecombination of target return counts in descending order and checking theresulting combination number, 0,, against a predetermined minimumacceptable plot quality, Q for the range of the target and the seastate. If this check is passed by a particular target, it is enteredinto a table for automatic acquisition, provided the track store is notfull to capacity; otherwise, the target is set up for display only forpossible manual acquisition of track. Once acquired, the target istracked automatically.

9 Claims, 26 Drawing Figures Red" lynchmnim Anmmu Commit-r DopplerPromo:

m gmuszoxm I 3,8191T4 Fig.2.

PAKENTEDAUBZOW i I warm sum as 0F 23;

3O Fig. 4. 32

J A U V Communications avlonlcs system system Digital computer :2

system Radar system 33 Radar Sig. proc. control unit Receiver s IExciter Beam steering and array 56 switch a Avionics Transmitter mission52 system Aft array g9 Fore array 1 a s I y Stbd array 5 fi i 46 I a fPort array 1 i l i J Fig. 50.

Port Stbd Aft Fora g' s 88 I t 1 86 n enna 46 7 Low Jim? 1 PhaseShifters 46 48 50 52 88 s 48 5o 52 r f f a r I 3 r 3 j Feed StructuresHigh :ower SLB Switch/Duplexer 84 L d "i R Transmitter W Receiver s'gnalDlgltal l l-u, PI'OGGSSOI' unit computer A I I L... .4

fl -TO RADAR CONTROL UNIT Exciter A 4 Ref. source (excitar) M 5 4PRIMARY RADAR SUBSYS TEM Pmtmsnwczm seam? SHEE? a? 23 Fore antennaReceiver output Port antenna Starboard anten na Transmitter input Aft 98 antenna la MTI double canoeller I (B n c C FromA/D n I i (A ,B) (B c)converter 10 bits sign Subtracter Subtracter bits" n n Y A sign I50 BI54 n v 1 (A a Sweep 1st Diff. storage. 4; store B (A B To 8 bitsdoppier sign accumulator Limiter I PATETEBBUBZOW $3M? 34 Digitalcomputer 40 Radar control unit Back up '36 scan program A Manual I r 7Input Radar data loglc buffer Receiver Exciter Radar Antennasynchronizer Computer High power & I34 4 switch 5| i Exciter Arrayswitches Timin Phase 9 shifter Signal processor I02 I06 "2 H6 H8 I20 l lI I l Double A/D Blank e MTl Q 0 Adaptive Coherent Noncoherent threshold104 integrator integrator a ilter selection Q Double 9 A/D Blankw q MTlL I L 1 I08 W A /D Doppler Processor no I SLB 1* I00 2 8 {VIH ClearRecvr z channel A L SGHSOF Fig6.

SIGNAL PROCESOR AND RADAR CONTROL UNIT ggiuszoim 3.831 @174 Fig. 90.

For next dwell R stop (range cell stop) signal processor A F (grounddoppler offset) 1 For this dwell To computer beam return p CL (clutterhave" Signal processor CT (subreport count) J (jam level) R (range) Fno. (filter no.)

S/C+N (signal/clutter'+ Noise) RADAR DATA BUFFER mmrnlu zmw 3,831,174

I m urea Fig. 9b.

1 EL 2 A F 3 R start 4 R stop 5 PR F Bits ID (array ID) 00 Fore 10 Aft 201 Strbd 11 Port M (mode) 0 Search 4 Test v 3 1 Verify 5 Sea State 2Track 3, 6 and 7 not defined Az (sine of Az angle) sin .088 i sin 55 12EL (sine of EL angle) sin .088 i sin 9.8 12

T (threshold) 7 .5 db 7 20 db 6 F (frequency) 0 31 5 AF (gnd doppleroffset) 3 Hz i 4000 Hz 12 PR F (pulse repetition freq) IPPS 1562-3125 12FP (fill pulses) v 1 pulse 8 13 4 B (no. of bursts) 1 burst '0 3 (N-1) 2R start (range cell start) 1 RC 0 5000 -13 R stop (range cell stop) 1 RC0 5000 t 13 RADAR CONTROL COMMAND FORMAT I Pmtmmauczmm 3,831,114

Fig. 9c.

2 RCT 3 4 Header Report 1 Header (one per dwell) F (frequency) 0 31 5 Az(sine of A2 angle) sin .088 i sin 55 12 EL (sine of EL angle) sin .088 isin 9.8 12

CL (clutter level) 0 31 5 ID (array ID) 00 Fore 10 Aft 2 01 Strbd 11Port SRCT (sub-report count) I 0 63 6 M (mode) 0 Search I 4 Test 3 1Verify v 5 Sea State 2 Track 3, 6 and 7 not defined (jamlevel/frequency) 0 7 3 PRF (pulse repetition frequency) IPPS 1562-312512 Report (N" per dwell) R (ambiguous range) I IRC 0 5000 13 S(signal/clutter noise) 2 db 8 40 db 4 A (amplitude) 3/8 db 0 db 9 F(filter number) 0 15 4' BEAM RETURN REPORT FORMAT PATENIEB 3,831 l 74 13 G? 23 268 Clock CL F 272 BL BL k 270 J Res Radar data 266 bufferCounter CL 274 r l L SUD 276 T Radar sync 250 T LPS w t I 278 275 D Iccs l Register 88y load T SDO Sweepl SD'i 'nterva v 252 register T 297296 Parallel 280 Delay 2 4 enter T TS CL 7 DEC Counter L 9 292 298 290 L3|2 Dem v r 302 1 DEC FP =0 2 3 D I T register Data 8 8y 0 SP sDi a 300i O DEC FP +16 B 30 T register 0 e 260 4 Scale of t coun er f 323 L 3 I8 Delay 262 T R start a DA register 332 C a SDR CL DE Counter '40 J 264334 X Ts R stpp register o EDR CL DEC Counter :0 Z w From synchronizerData buffer MENTED auszo ran LPs -1 sun To 5-bit phase Shifter Antennaphase computer Element no. 1

Fig. '11,

'--' Array switch MENIEU AUG 2 0 lfll l MAAOOO Initialize to process 3dwells of data Dwell 1 no returns sum 1e W 23 Dwell 2 no returns Fig.

Yes

Dwell 3 no returns 405 s P Pass 1 Pass 2 Pass 3 X X0 Pmnte' 1 X 0 2 x xPointer to dwell 3 buffer buffer 0 buffer Y Polnter to dwell 2 Y0pointer to dwe|| 3 Y O ("u") buffer buffer Z Pointer to dwell 3 2 0 Z 0(null) buffer XE Dwell 3 end of XE Dwell 1 end of buffer XE Dw ll 2 endof buff r buffer YE Dwe" 2 end f b ffer YE Dwell 3 end of buffer R Dwe"3 f ld ZE Dwell 3 end of buffer R Dwell 2 foldover range R Dwell 1foldover range range R Dwell 3 foldover R Dwell 2 foldover range range RDwell 3 foldover range Rx processed Yes Set X to next dwell X return Endof dwell X table (X XE) Pitlllfniuczolsn Resolve range ambiguity: Ruunambiguous/ ambiguous range CxCyCz plot combination (primary) Calculate1st multipath delay: \R function of range Ru Multipath detect and count:CxCyCz plot combination (primary multipaths) Cross correlation this plotvs track store:

Flag correlated or not correlated Auto acquisition quality check:passtrack candidate. not passdisplay of plot Pass pass 1 Auto acqui.candid.

= none Yes t l Sort auto acqui.

track candidates Param, no. 1 0

into decending (indicates no order. auto acqui. Param 1 no. of entriescandidates) 7 Param 2 address of table ATENYEU 2 01974 RRAODO a,Initialize primary (multipath counters Cx 1 Cy O Cx O Pass Yes x N (R xXF . No Yes RY NY (R ii tolZR s Z 0 (null) b (any nonprocessed v) 426 IYes g N0 433 v Cy 1 431 Ru; Rx N (Rd RX Nx XF Ru U R x processe N X RVProcessed Hz N2 zF i max o (ambiguous (any nonrange) processed 427 Yes 20 (null) Yes CZ =1 Ru Rx N (R Rx processed R2 processed No Ru R1 N1 (R itoliR No Q 7 (any non-- processed Rz's) Return Fig. 15.

Calculate look-up range index R Ru ARL where A R is look-up range stepsize quotient Use look-up range (R to pick up- R from multipath delay vsrange look-up table Return

1. An airborne radar system for searching and tracking airborne targetsover large bodies of water comprising a radar system having a searchbeam with a selected number of dwells using a different one of aselected number of staggered pulse rEpetition frequencies, PRF''s, foreach dwell in order to resolve the range ambiguity of targets, saidradar system including, antenna means, a radar control unit includingbuffer means coupled to said antenna means, transmitting means coupledto said radar control unit and to said antenna means, and receivingmeans including signal processing means coupled to the buffer means ofsaid radar control unit, a computer system coupled to the buffer storagemeans of said radar control unit for more positive identification of anairborne target for automatic acquisition of the target in a track storeof said radar system, including computer means for resolving the rangeof each primary return of potential targets by correlating returns ofsaid selected number of dwells, computer means for counting the numberof returns received for each primary return at a given range in eachdwell, said number constituting a count of 1 for the primary target, acount of 2 for the primary return and a first multipath return havingone reflection from the surface of said body of water, and a count of 3for the primary return and a second multipath return having tworeflections from said surface, computer means for determining thequality of each potential target as a function of three count numbers ofthe primary and multipath returns received from said selected number ofdwells, and computer means for setting up an entry into an automatictrack candidate table of target data for each potential target having aquality equal to or greater than a predetermined minimum, said targetdata including the PRF of the search dwell having the largest multipathreturn count number from which the quality is determined.
 2. Apparatusas defined in claim 1 wherein said signal processing means comprises adoppler processor to yield relative radial velocity of a target, andwherein said computer means for counting the number of returns receivedfor each primary return at a given range in each dwell includes meansfor selecting only those returns having substantially the same relativeradial velocity.
 3. Apparatus as defined in claim 2 wherein saidcomputer means for determining the quality of each target is comprisedof computer means for forming a three-digit number from said countnumbers in descending order of magnitude, each digit having a value of0, 1, 2 or 3 depending upon whether in a particular dwell with which thecount number is associated there was no primary return, a primary returnand no multipath returns, a primary return and a first multipath return,or a primary return, a first multipath return and a second multipathreturn.
 4. Apparatus as defined in claim 3 wherein said predeterminedminimum is a function of sea state and slant range of the target fromthe sensor of said radar system, and said computer means for setting upan entry into an automatic track candidate table includes means fordetermining said minimum as a function of target slant range and seastate.
 5. An airborne radar system used to search for airborne targetsover a large body of water and to track targets found, said systemcomprising radar means having buffer storage means for storing beamreports and beam parameters, a transmitter for transmitting a search andtrack beam in selected directions, said beam for searching including aplurality of dwells, each dwell consisting of radar pulses transmittedat a unique pulse repetition frequency, PRF, a computing systemincluding input-output means coupled to said buffer storage means, saidcomputing system including computer means for resolving the rangeambiguity of each dwell return and including means for identifyingreturns in different dwells which relate to the same target, means forstoring an unambiguous range for each potential target having returns inat least two dwells and for storing an ambiguous range for eachpotential target having returns in only one dwell, computer meAns fordetermining the maximum delay time in terms of range, Delta RM1, for afirst multipath return from each potential target where the firstmultipath return includes a single reflection from the surface of saidbody of water in the path of a radar pulse from said radar system to thetarget and return, computer means for determining the presence of afirst multipath return for each potential target in each dwell at arange Ri by checking all other returns of the same dwell for a return ata range RiM1 which satisfies the condition RiM1 - Ri < Delta RM1, andfor each first multipath return found determining the presence of asecond multipath return for each potential target in each dwell, wherethe second multipath return includes two reflections from the surface ofsaid body of water, by checking all other returns of the same dwell fora return at a range RiM2 - Ri 2(RiM1 - Ri) + or - tol, where tol is atolerance value, thereby producing a plot combination count for eachpotential target of unambiguous or ambiguous range showing the countnumber of primary and multipath returns received in the three dwells,said count number being zero in two dwells for a potential target ofambiguous range, computer means for correlating each potential target ofunambiguous range with all other targets being tracked which are in thesame direction as the search beam, and to mark each potential target ofunambiguous range which correlates with a target being tracked with anidentifying flag, computer means for establishing a plot quality foreach potential target from said plot combination count numbers bycreating a plot quality number consisting of said count numbers of thepotential target for all dwells arranged in descending order ofmagnitude, computer means for determining a minimum plot qualityrequired as a criterion for automatic acquisition of a target as afunction of current sea state and target data comprised of target range,and computer means for determining which potential targets have a plotquality which satisfies said minimum plot quality criterion forautomatic acquisition by said radar system into a target track datastoring means.
 6. Apparatus as defined in claim 5 wherein said radarsystem includes a radar signal processor coupled to said buffer storagemeans comprising a doppler processor to yield relative radial velocityof a target, and wherein said computer means for counting the number ofreturns received for each primary return at a given range in each dwellincludes means for selecting only those returns having substantially thesame relative radial velocity.
 7. Apparatus as defined in claim 6including computer means, responsive to a determination made for a giventarget that said minimum plot quality criterion has been satisfied, forsetting up an entry into an automatic acquisition track candidate tableinformation which will permit said radar system to automatically acquiresaid given target.
 8. Apparatus as defined in claim 7 wherein saidinformation in said entry into an automatic acquisition track candidatetable includes the PRF of the dwell having the highest multipath returncount, and further comprising means coupled from said automatic trackcandidate acquisition table to said buffer storage means for permittingthe radar system to automatically initiate acquisition of the target fortracking using that PRF which has the highest probability of providing averify beam return from said given target.
 9. Apparatus as defined inclaim 8 including a display means and computer means responsive to agiven target processed of ambiguous range for setting up a display ofsaid target in the direction of said radar beam and at said ambiguousrange, thereby permitting a radar operator to take steps to manuallyacquire said given target.